1. Field of the Invention
The present invention relates to a wafer transfer device for a semiconductor device fabricating system and, more particularly, to a revolving frog leg type wafer transfer device for a semiconductor fabricating system.
2. Description of the Prior Art
Techniques for a semiconductor device manufacturing process for manufacturing a very small semiconductor device with a high accuracy must be capable of transferring a semiconductor substrate from one processing chamber to the next processing chamber for subsequent processing without changing the quality of the surface of the semiconductor substrate. Nevertheless, in transferring a semiconductor wafer from one chamber after completing a process, such as an etching process, in the chamber to the next chamber for the next process, such as a CVD process, in a practical process, the semiconductor wafer is exposed to the atmosphere. Consequently, oxide films are formed by natural oxidation over portions of a wiring film exposed to through holes. In most cases, such oxide films must be removed because the oxide films increase contact resistance. The removal of the oxide films formed by natural oxidation requires immersing the semiconductor wafer in an etchant and washing the semiconductor wafer to wash off the etchant. Consequently, the process needs additional steps and the throughput of the process is reduced. To deal with such problems, a multichamber semiconductor fabricating system comprising a plurality of processing chambers connected by gate valves has been developed and proposed in, for example, NIKKEI MICRODEVICES, Oct., 1989, pp. 34-39.
The multichamber semiconductor device fabricating system needs a wafer transfer device capable of smoothly sending a semiconductor wafer into and smoothly taking out the semiconductor wafer from each processing chamber to carry out efficiently the semiconductor fabricating process. FIG. 4 shows a prior art frog leg type semiconductor wafer transfer device proposed for such a purpose.
Referring to FIG. 4, a bottomless, cylindrical member 1 has a flange at its upper end (only the flange is shown in FIG. 4) fastened directly or indirectly to a horizontal shielding plate 2 so that the cylindrical member is held in a vertical position on the shielding plate 2, a bottomless, outer rotary tube 3 (only its upper end is shown in FIG. 4) is supported on the cylindrical member 1 by a magnetic fluid, an inner rotary tube 4 is inserted in and supported by a linear motion bearing on the outer rotary tube 3, and a bellows 5 is fixed at its lower end to the upper end of the outer rotary tube 3 and at its upper end to the circumference of the upper end of the inner rotary tube 4. The inner rotary tube 4 rotates together with the outer rotary tube 3 when the outer rotary tube 3 is rotated by driving means, such as a motor, not shown, and is capable of being moved vertically by lifting means, not shown. A driving shaft 6 is inserted in and supported by a magnetic fluid, not shown, on the inner rotary tube 4, and a driving gear 7 is fixed to the upper end of the driving shaft 6 projecting upward from the upper end of the inner rotary tube 4. A link 8a is joined at its one end to the driving gear 7. A shaft 9 is attached to the upper end of the inner rotary tube 4, and a driven gear 10 is supported on the shaft 9 so as to engage the driving gear 7. A link 8b having a length equal to that of the link 8a is joined at its one end to the driven gear 10. The driving shaft 6 is rotated by a motor, not shown; consequently, the link 8a turns in the same direction as a direction in which the driving shaft 6 is rotated and the link 8b joined to the driven gear 10 turns in the opposite directions. Thus, the links 8a and 8b turn respectively in opposite direction.
One end of a link 11a is joined pivotally to the other end of the link 8a, and one end of a link 11b is joined pivotally to the other end of the link 8b. Mating gears 12 are provided on the other ends of the links 11a and 11b. Thus, the links 8a, 8b, 11a and 11b form a frog leg type linkage. A wafer support plate 13 is held on the other ends of the links 11a and 11b by the shafts of the gears 12. The wafer support plate 13 is capable of supporting a semiconductor wafer 14.
The situation of the frog leg linkage consisting of the links 8a, 8b, 11a and 11b can be changed by turning the outer rotary tube 3, and the frog leg linkage can be stretched or contracted by rotating the driving gear 6 to advance or retreat the wafer support plate 13. The wafer support plate 13 can vertically be moved by vertically moving the inner rotary tube 4 by the lifting means. The wafer support plate 13 must be moved vertically up and down in a predetermined range in transferring the semiconductor wafer 14 from a wafer transfer station in the chamber to the wafer support plate 13 and vice versa.
Processing chambers 15 to which the semiconductor wafer 14 is supplied and from which the semiconductor wafer is taken out by the wafer transfer device are arranged around the wafer transfer device. In FIG. 4, only one of the processing chambers 15 is indicated by alternate long and two short dashes lines for simplification. Shown also in FIG. 4 are gate valves 16 provided between the wafer transfer device and the processing chambers 15, respectively, gates 17 through which the semiconductor wafer 14 is passed, wafer detectors 18 for detecting the semiconductor wafer 14, and a discharge port 19.
When the frog leg linkage of the wafer transfer device shown in FIG. 4 is contracted in a state where the links 8a, 8b, 11a and 11b thereof are aligned with a straight line with the front end of the frog leg linkage located above the driving gear 6 as shown in FIG. 6, it is difficult to stretch the frog leg linkage in such a state, because a large torque must be applied to the links 8a and 8b in stretching the frog leg linkage from the fully contracted state similarly to folding the unfolded blade of a jack-knife. Such a phenomenon is called a jackknife phenomenon. If the frog leg linkage is locked in the fully contracted state by the jackknife phenomenon, the chamber containing the wafer transfer device and evacuated to a high vacuum must be opened and the frog leg linkage must be operated by hand so that the frog leg linkage can be stretched. Once the chamber is opened to expose the interior thereof to the atmosphere, the inner surface of the walls of the chamber is wetted by moisture contained in the atmosphere and the chamber must be evacuated to the high vacuum, which requires a long time and the multichamber apparatus is held inoperative for several hours or several days. Consequently, the operating ratio of the production system including the multichamber semiconductor fabricating system is reduced and thereby the manufacturing cost of the semiconductor device is increased. Therefore, the jackknife phenomenon must unconditionally be avoided.
The inventors of the present invention attempted to construct the frog leg linkage and the associated mechanisms so that the front end of the frog leg linkage may not be located above the driving gear 7 when the frog leg linkage is fully contracted as shown in FIG. 6. For example, the range of rotation of the driving gear 7 was narrowed.
However, the attempt caused another problem that affects adversely to the improvement of the throughput.
Originally, the frog leg type wafer transfer device is capable of contracting the stretched frog leg linkage so that the wafer support plate 13 passes over the driving gear (hereinafter, such a movement of the wafer support plate 13 will be called "transition past the dead point") and of stretching the frog leg linkage in the opposite direction. If the frog leg linkage can be stretched and contracted in opposite directions in such a manner, the stroke of the wafer support plate 13 is twice the length of the frog leg linkage corresponding to a length twice the length of the links, and the wafer support plate 13 can be moved past the driving gear 7 (hereinafter referred to as "dead point") in opposite directions. If the frog leg linkage is capable of such motions, the semiconductor wafer 14 can be transferred from one chamber to a diametrically opposite chamber only by the action of the frog leg linkage without turning the frog leg linkage. Such capability of the frog leg linkage improves the throughput effectively. Furthermore, if the wafer support plate 13 is provided with two wafer mounts to support two semiconductor wafers at a time, the throughput can further be improved. However, if the range of rotation of the driving gear 7 is narrowed so that the frog leg linkage cannot be retracted in a fully retracted state as indicated by continuous lines in FIG. 6 to avoid the jackknife phenomenon, the wafer support plate 13 is unable to move past the dead point, the stroke of the wafer support plate 13 is reduced by half, it is impossible to transfer the semiconductor wafer 14 from one processing chamber to another processing chamber without turning the frog leg linkage, the capability of supporting two semiconductor wafers of the wafer support plate 13 becomes almost meaningless, and the significant improvement of the throughput cannot be achieved.
The inventors of the present invention attempted to reduce torque necessary for driving the fully contracted frog leg linkage to make the frog leg linkage escape from the dead point, by combining an auxiliary escape mechanism with the frog leg linkage. As shown in FIG. 5, the auxiliary escape mechanism comprises a pulley 20 fixed to the driving gear 7, a pulley 21 having a diameter half times that of the pulley 20 and rotatably supported on a joint joining the links 8a and 11a, and a belt 22 extended between the pulley 20 and the pulley 21. Torque to be produced by an escape motor for driving the auxiliary escape mechanism is expressed by: EQU M=2FL.sup.2 /r
where M is the torque to be produced by a motor, F is the force necessary for moving the wafer support plate 13 out of the dead point above the driving gear 6, L is the length of the links 8a, 8b, 11a and 11b, and r is the radius of the pulley 20, i.e., the diameter of the pulley 21.
The auxiliary escape mechanism, however, has problems that the required torque M increases with the length L of the links, it is possible that the belt 22 is broken if the required torque M is very large, and it is possible that the step out of the motor occurs.
Although the inventors of the present invention intended to reduce the torque M necessary for moving the frog leg linkage out of the dead point by increasing the diameter of the pulley 20, the inventors imagined that the diameter of the pulley 20 may be limited by the center distance between the driving gear 6 and the driven gear 10 because the pulley 20 is fixed to the driving gear 6 engaging the driven gear 10. Thus, it was considered that there is a limit to the reduction of the torque by increasing the diameter of the pulley 20 and it is difficult for the auxiliary escape mechanism to overcome the difficulty.